The present invention relates to sheet feeding and, in particular, to the feeding of sheets, such as paper sheets, one-at-a-time from a stack and detecting double sheets.
It is known to feed sheets sequentially, i.e., one-at-a-time from a vertical stack, e.g., in sheet-printing and sheet-coating machines. For example, U.S. Pat. Nos. 7,125,014 and 7,207,558 disclose a sheet feeding apparatus employing suction belts which grip the upper surface of a top sheet in the stack and advance the sheet. The disclosures of those patents are incorporated by reference herein.
It is, of course, desirable to prevent the feeding of double sheets, defined herein as arising when a second sheet adheres itself to the underside of the sheet above it in the stack, e.g., due to static friction. Such double sheet feeding is undesirable, especially in the case where paper sheets are fed to a sheet-coating apparatuses in which the coatings are cured by passing the coated sheets beneath a heater which emits intense heat. In the case of double sheets being fed, the extra bottom sheet can become dislodged from the top sheet and possibly immobilized beneath the heater, whereupon overheating of the immobilized sheet can produce a fire.
Efforts to sense the feeding of double sheets are known, such as disclosed in U.S. Pat. No. 4,420,747 in which sheets are passed successively through the nip of a roller pair, a lower one of the rollers being driven about a fixed axis, and a top one of the rollers being freewheeling and vertically movable. The passing of a sheet through the nip causes the top roller to be displaced upwardly. That roller displacement is sensed by a transmitter which sends a signal to electric evaluator circuits. When double sheets pass through the nip, the greater thickness of the double sheets produces an increase in the roller displacement, which is sensed by the evaluator circuits, and an appropriate warning signal is produced. The disclosure of this patent is incorporated by reference herein.
Despite the precaution heretofore taken in the art to prevent the feeding of double sheets, room for improvement remains. For example, in the case of the roller pair disclosed in afore-mentioned U.S. Pat. No. 4,420,747, it will be appreciated that expensive rollers of high-precision manufacture and positioning are required in order to be able to reliably detect the minute difference in sheet thickness between a single sheet and double sheets, especially when very thin paper sheets are being fed. The reason is that if rollers of imprecise positioning or shape, e.g., of eccentric or out-of-round shape, are used, displacements of the freewheeling roller can occur just because of such imprecise positioning or shape. Since the difference in roller displacement between the feeding of single sheets versus double sheets is minute, the creation of such false displacements can produce unreliable results.
It would be desirable to provide a sheet feeding system which minimizes the chances for double sheets to be fed from a stack, and/or maximizes the chances for the feeding of double sheets to be detected along a feed path.